Biaxially stretched polyamide resin films using polyamides such as nylon 6 and nylon 66 are excellent in the mechanical properties such as tensile strength, sticking strength, pinhole strength and impact-resistant strength, and additionally, in gas-barrier property and heat resistance. Accordingly, these films are used in wide applications.
When these biaxially stretched polyamide resin films are used as packaging materials, these biaxially stretched polyamide resin films are usually used as front substrates of laminated films, and are in many cases free from direct contact with contents as the packaged articles. Accordingly, the behavior of the low molecular weight compounds in the biaxially stretched polyamide resin films has not been much mentioned yet.
However, when the films are heated in the production step thereof or in the processing step such as a step of laminating or printing, the low molecular weight compounds contained in the polyamide resin films are deposited on the surface of the films as the case may be. As the current packaging techniques are sophisticated, this phenomenon comes to be an issue impossible to be left out of consideration because troubles, for example, ascribable to this phenomenon are caused.
For the purpose of coping with the issue, there have been proposed polyamide resins, in each of which the molecular weight of the constituent monomer unit is large, such as nylon 11 and nylon 12 or copolyamide resins mainly composed of nylon 11 and nylon 12 (JP4-325159A). Alternatively, a copolymerized polyamide resin between 1,6-hexanediamine and sebacic acid has also been proposed (JP2001-328681A). However, these are specific polyamides, and are high in price and low in versatility. Consequently, strongly demanded are films in which highly versatile nylon 6 and nylon 66 are used and the monomer contents are low.
Even if the low molecular weight compounds such as the unreacted monomers and oligomers are removed from a polyamide resin at the stage of being chips prior to film molding, remelting of the resin chips with a melt extruder or the like regenerates the low molecular weight compounds, and consequently the low molecular weight compounds remain in the film to degrade the quality of the film. In particular, a polyamide in which caproamide is the main repeating unit thereof has a characteristic that the low molecular weight compounds such as the monomers tend to be more easily generated than in polyamides formed of a dicarboxylic acid and a diamine and the content of the low molecular weight compounds is large.
In general, when the terminal group concentration of a polyamide resin is higher, the regeneration amount of the low molecular weight compounds such as the monomers at the time of remelting tends to be larger. For the purpose of coping with this problem, there has been developed a polyamide in which a compound capable of reacting with the carboxyl terminals or the amino terminals of the polyamide is added. Specifically, there has been proposed a method in which an organic glycidyl ester is reacted with the carboxyl groups and the amino groups of the polyamide (JP10-219104A). However, in this method, when the organic glycidyl ester and the polyamide chips are dry blended and then melt-kneaded in an extruder, the organic glycidyl ester is allowed to react with the terminal groups of the polyamide. Additionally, in this method, it is difficult to perform uniform mixing in the dry blending step prior to film molding. Consequently, such non-uniform mixing offers a cause for the composition variation. Thus, it is difficult to obtain a polyamide having a uniform terminal group concentration, and moreover, the dry blending step itself is unsuitable for films involving large melt extrusion amounts. Additionally, in this method, the content of the low molecular weight compounds after the melt molding is still large, and the reduction amount of the low molecular weight compounds is not sufficient.
On the other hand, there has been proposed a method in which the terminal amino groups of a polyamide resin are blocked with a dicarboxylic acid anhydride (JP2005-187665A). However, the amount of the regenerated monomers at the time of melting is still as large as 0.27 to 0.75% by mass, and it is difficult to sufficiently reduce the amount of the low molecular weight compounds contained in the polyamide resin film.
Additionally, there has been proposed a nylon 6 resin in which a piperidone compound is chemically bonded to the polymer chain or the terminals of the polymer (JP2001-081189A). In this resin, the generation of caprolactam resulting from the melting under a reduced pressure is reduced. However, in the film formation step generally performed under normal pressure, the oligomers generated when the resin is melted are hardly discharged to outside the system, and hence the oligomer reduction effect is insufficient.
As described above, with respect to the low molecular weight compounds of the polyamide resin, various proposals have been offered. However, no documents have mentioned the low molecular weight compounds in the nylon products including a polyamide (nylon MXD) composed of xylylenediamine and an aliphatic dicarboxylic acid having 4 to 12 carbon atoms.
In particular, films formed of a blend resin composed of nylon 6 or nylon 66 and nylon MXD, or films having a multilayer lamination configuration constituted with nylon 6 or nylon 66 and nylon MXD are capable of imparting additional functionalities to a film formed of nylon 6 resin or nylon 66 resin, and hence, under the circumstances as described above, such films have come to be widely used as gas-barrier films, straight-cut films, easy-tear films, contractive films and the like.
However, nylon MXD-containing films are abundant in the content of the low molecular weight compounds, and hence cause problems in the production step or the processing step. Such films are larger in the amounts of the low molecular weight compounds deposited outside the films as compared to, for example, nylon 6 resin films, and hence are strongly required to be improved for such deposition.
As a countermeasure against the deposition of the low molecular weight compounds, WO 2008/075461A1 by the present applicants proposes, with respect to the nylon 6 resin, a method for efficiently removing the low molecular weight compounds such as monomers by using heated water. However, this method is unable to effectively remove the low molecular weight compounds from products made of nylon MXD-containing nylon resins.